where \(\delta (z)\equiv \delta [\eta (z)]\) .
A useful parametrization of this effect, which catches the full redshift dependence in terms of just two parameters \((\Xi _0,n)\) , is obtained writing [1]},
\(\frac{d_L^{\,\rm gw}(z)}{d_L^{\,\rm em}(z)}\equiv \Xi (z)=\Xi _0 +\frac{1-\Xi _0}{(1+z)^n} \, .\) 
